Throughout this application various patents and publications are referenced and citations are provided in parentheses. The disclosure of these patents and publications in their entireties are hereby incorporated by reference into this application in order to more fully describe the state of the art to which this invention pertains.
Filters, membranes, beads, and other porous and nonporous articles are widely used for separating and concentrating substances present in liquid environments. For example, filters and membranes have utility in applications involving reverse osmosis, ultrafiltration, microfiltration, pervaporation and dialysis. In general such applications rely upon pressure to effect separation or concentration. Therefore, it is important that the filter or membrane material maintains its physical integrity and desired properties under pressure.
It is well-known that polymers are useful raw materials for the production of filters, membranes, and other separation and chromatography media. Typically, the polymers useful for the fabrication of rigid porous articles tend not to be dissolved by, or swell in, water and are commonly referred to as being hydrophobic in nature, i.e., acrylonitrile polymers or copolymers. Unfortunately, the polymeric qualities which render crystallinity and physical strength to the formed porous articles also cause adsorptive interactions during separation or concentration work. As a consequence, the porous article becomes fouled by materials in the feed stream to be separated or concentrated. Such fouling is a major problem because the formation of a fouling layer upon the porous article's surface interferes with the article's efficiencies and specificities thereby necessitating frequent and costly cleaning treatments. Other types of polymeric compositions which tend not to exhibit adsorptive interactions during separation work also tend to lack the necessary physical strength for pressure driven separations. These compositions are pressure sensitive and can be readily compressed and distorted by applied pressure. Agarose and polyacrylamide are examples of such compositions and are described as hydrated gels because throughout their structures they are hydrophilic in nature, i.e. interacting with water and swelling thereby.
Attempts have been made to overcome the disadvantages of hydrophobic-type polymeric compositions by chemically modifying the surface of formed porous articles. Linder et al., U.S. Pat. Nos. 4,584,103 (1986) and 4,477,634 (1984), describe methods of increasing the pressure stability of a polyacrylonitrile-containing membrane by reaction with hydroxylamine followed by additional steps including reaction with a poly-functional oligomer and a compound containing at least one ionic group. A disadvantage of this method, however, is that the resulting modified membranes contain charged groups, thereby making them unsuitable for some applications. Additionally, the methods suffer from defects such as the need for expensive reagents and poor control over the extent of modification.
Sano et al., U.S. Pat. No. 4,265,959 (1981), describes a method for the preparation of semipermeable membranes which comprises sulfonating porous membranes of acrylonitrile polymers. In the disclosed method, a porous membrane is exposed to a gasous sulfonating agent, i.e. sulfuric anhydride, under pressure. The resulting modified membrane is claimed to possess superior chemical, mechanical and thermal properties due to cross-linking of its surface molecules and its increased hydrophilic character. However, the Sano et al. method suffers from similar defects as described above; that is, the need for expensive reagents and poor control over the extent of modification. In fact, the disclosure notes that if the whole membrane is sulfonated it becomes brittle.
In Sano et al., U.S. Pat. No. 4,147,745 (1979), a surface-modifying method is disclosed which comprises exposing a membrane of acrylonitrile-type polymers to a plasma. The resulting membrane is claimed to have a surface whose polymer molecules are cross-linked thereby increasing its physical strength. Nakanishi, U.S. Pat. No. 4,501,785 (1985) relates to a method of hydrophilizing a porous membrane made of a polyolefin, i.e. polyethylene, by coating the surfaces which define the pores with polyethylene glycol. These methods also involve expensive procedures and the Nakanishi method, additionally, is limited to membranes whose pores can accommodate the polyethylene glycol molecule.
The subject invention provides membranes and other porous articles having superior physical integrity to withstand pressure driven separations and hydrophilic surfaces to prevent fouling. The procedures of the present invention comprises a chemical reaction between nitrile groups of hydrophobic-type polymer and an aldehyde to produce hydrophilic amide groups only on the surface of a formed article. The fundamental chemistry of this reaction is well-known in the art (see e.g., Magat, E. E., et al., J. Am. Chem. Soc., 1951, 73: 1028-1037; Mowry, D. T., U.S. Pat. No. 2,534,204 [1950]). Mowry, British Patent No. 677,516 (1948) describe a method utilizing this chemical reaction for the synthesis of "nylon" type polymers. However, the Mowry method produces polymers having the resulting amide groups as part of the polymer backbone and, therefore, articles formed from these polymers have the amide groups throughout their structures. In contrast, the present invention provides a method of reacting the nitrile groups pendent to the polymer backbone in preformed articles. Additionally, the inventive method enables one to control the reaction rate so that only the surface of the article contains amide groups thereby providing fouling resistance to the surface while maintaining the physical strength of the article.